The negative effect of ceria on the propene selectivity for isopropanol decomposition over phosphated and phosphate-free ceria/alumina catalysts
© Khalaf; licensee Springer. 2013
Received: 7 October 2013
Accepted: 11 November 2013
Published: 20 November 2013
Consequences of loading level of ceria (5, 10 and 20-wt%) on the structural, textural and surface acid–base properties as well as the impacts on catalytic activity towards isopropanol decomposition on pure and phosphated alumina is described. The obtained results indicated that composites mainly kept γ–alumina structure and the addition of ceria and/or phosphate has no effect on the bulk structure of alumina. Textural characteristics show that the phosphated composites have higher surface area than phosphate free samples. Pyridine adsorption as followed by FTIR indicated the presence of Lewis acid sites with different strengths. Moreover, results of gas phase decomposition of isopropanol show a systematic inhibition in propene selectivity based on ceria ratio.
KeywordsTexture Mesoporous Acidity Activity Decomposition
Active alumina, as one of the most common catalyst support, has a good performance in catalysis applications due to its physical and chemical properties (Levy et al. 1968). Alumina has various properties such as high specific surface area, surface acidity and defects in their crystalline structure that are the key factor in the wide range of applications. One of the most important factors in alumina utilization is thermal stability, which is a very important issue for processes running at high temperature (Chen et al. 2001). Therefore, the addition of some additives can improve the thermal stability of alumina catalysts, hold the surface area and improve the surface acidity (Shinjoh 2006; Ozawa 2006; Ozawa & Nishio 2004; Ersoy & Gunay 2004). Phosphates have been claimed to act as a support stabilizer (Gishti et al. 1984; Abbattista et al. 1990; Khalaf et al. 2007). Gishti et al. (1984; evidenced a stable effect of phosphate by studying the phase transitions of alumina in the range 673–1323 K. However, the surface area decreased with the increasing content of phosphate in alumina, even less than the blank sample after calcinated at 1073 K. Khalaf et al. 2007 also proved no surface area stabilization effect from phosphates in the case of the transition phase (spinel) aluminas.
Ceria possesses versatile acid–base properties, depending on the nature and temperature of the pretreatment. It may have high number of basic sites of weak or medium strength (Binet et al. 1999). Martin and Duprez found the following scale for the density of basic sites of some metal oxides CeO2 > MgO > ZrO2 > Al2O3 > SiO2 (Martin & Duprez 1997). Binet et al. (1999) also observed that ceria can chemisorb CO or pyridine, but the band positions strongly suggest that the Lewis acidity of ceria is significantly lower than that of zirconia or titania. In contrast to Lewis basicity, the Lewis acidity would decrease upon reduction of ceria.
Ceria and ceria-based materials has been widely employed as automotive three-way catalysts (TWC) for reducing the exhaust pollutants, fuel cell processes, oxygen permeation membrane systems, exhaust combustion catalysts, and catalytic wet oxidation (Yao & Yao 1984; Dos Santos et al. 2008; Bera et al. 2003; Khalil 2007). Cerium oxide can maintain large amounts of oxygen through the easy transition between Ce(IV) and Ce(III). This property is very useful in catalysis especially in oxidation reactions (Guimaraes et al. 2003). Nevertheless, pure ceria is poorly thermostable and undergoes rapid sintering under high temperature conditions, which leads to loss of oxygen buffer capacity and deactivation of the catalysts. The combination of two metals in an oxide can lead to novel structural and electronic properties of the final oxide, consequently modifying its catalytic activity and selectivity. Ceria-alumina mixed oxides are widely used as catalytic materials (Walter & Oldfield 1989; Niu et al. 1999; Sanchez-Valente et al. 2004).
In the present work, ceria-alumina as well as phosphate alumina modified with ceria have been prepared and characterized by several tools to gain an overview on the surface properties of these catalysts. In addition, the catalytic activities towards isopropanol decomposition for these composites have been explored.
Results and discussion
The thermal events of ceria-alumina, xCeAl, and phosphate ceria-alumina, xCeAlP, samples are shown in Figure 1. It is cleared that, the increasing in the ceria loading levels resulted in increasing in the mass loss % (39.3, 50.0 and 54.0% for 5CeAl, 10CeAl and 20CeAl samples, respectively). This is logically, due to ceria losses mass higher than alumina. As for phosphated samples, one can noticed that there is no change in the mass loss % by the comparison between phosphated and phosphate-free samples. Thus, the phosphate ions have slightly effect on the mass loss % of phosphate samples. Only the main difference between xCeAl and xCeAlP is in the wider extended region for phosphated samples.
DTA results for all samples show two endothermic peaks, the first one appeared at low temperature (about 330 K) due to the elemination of adsorbet water, and the other one appeared at temperature higher than 480 K due to the decomposition of samples.
X-ray diffraction (XRD)
Nitrogen adsorption-desorption data
Data obtained from isotherms and BET plots of the samples under testing are cited in Table 1. From these data, it is clear that pure alumina sample has higher specific surface area (SBET = 187 m2g-1) than pure ceria (SBET = 12 m2g-1) which agree with previous data (Gishti et al. 1984). The addition of ceria into alumina, xCeAl, resulted in a gradual decrease in the surface area for the samples (SBET = 117, 53 and 50 m2g-1 for 5CeAl, 10CeAl and 20CeAl, respectively). For phosphated samples, there is a slightly effect of phosphate on the value of specific surface area in which the SBET values for xCeAlP are higher than that of xCeAl. These data agreed with those obtained from XRD results, which complied in Table 1, in which the variation in crystallite size has an effect on the specific surface area (Khalaf 2009). Hence, the noticeable increase in the SBET values for xCeAlP after phosphate addition could be attributed to the decrease in the crystallite size rather than any modification in the pore structure of the pure alumina. This result agrees with what has been reported in the literature (Larese et al. 2004).
PSD curves of the samples under study are illustrated in Figure 4. Examining these curves reveals that the addition of ceria and/or phosphate on to alumina causes a development in the porosity. In which, all samples are in micro-meso range. Moreover, the amount of mesopores in the phosphate-free samples is higher than that of phosphate samples.
FTIR spectra of adsorbed pyridine
Spectra taken from Py adsorption on 5CeAl, 10CeAl and 20CeAl samples at 300 K, show common feature of a gradual weakness in the intensity of the bands characteristic of the pure alumina. Py adsorption over phosphated alumina modified with ceria with different loading levels show similar spectra with phosphate free samples, so we only mention only one (xCeAl samples) to prevent the recurrence.
Conversion% of isopropanol as well as propene and acetone selectivities at 470 K
The results obtained from decomposition of isopropanol on γ-Al2O3 (Table 2) display mainly conversion of isopropanol to propene, whereas the formation of acetone is found to be very low (3.5%). The obtained results indicated that the selectivity of propene is 96.5%, thus, alumina surface have strong acidic sites, which is evident from the higher dehydration process than dehydrogenation, these results agree with that in literature (Khalaf et al. 2007). Alumina surface is known to acquire strong Lewis acid sites, which explain the high activity towards the dehydration pathway (Waqif et al. 1992). The results for pure ceria show that the dehydrogenation (87.5%) of isopropanol to acetone is higher than the dehydration (12.5%) process indicating the basicity of ceria surface. Ceria modified alumina samples, xCeAl, show that the incorporation of ceria into alumina resulted in a decrease in the propene selectivity from 96.5% for alumina to 74.4% for 20CeAl. This descending in propene selectivity% is analog with ascending of acetone selectivity% and could be attributed to the basicity of ceria. (Zaki et al. 1997) reported that ceria surface has Lewis base sites assuming a strong nucleophilic reactivity.
At the same time, the conversion of isopropanol and the selectivity towards propene formation for the phosphate catalysts, xCeAlP, is higher than that for the phosphate free catalysts. Knowing that isopropanol dehydrate to propene over acidic catalysts, this could explain such behavior of the phosphated catalysts. Furthermore, the obvious increase in the catalytic activity (% conversion) of the phosphated catalysts than phosphate free samples is in accordance with the data obtained from surface area measurement, in which the SBET for xCeAlP > SBET for xCeAl.
In conclusion, the addition of ceria and/or phosphate onto alumina with different loading levels causes a modification in the surface texture and porosity. The results show also that the ceria added to alumina is well dispersed on the surface of alumina and has no effect on the phase structure of pure alumina (γ-Al2O3 structure). Alumina, ceria and ceria-alumina composite show Lewis acid sites with different acidity strengths. Isopropanol decomposed to mainly propene by pure alumina and the addition of ceria decreases the amount of propene selectivity% and increases the acetone selectivity%, this is due to the basicity of ceria. Moreover, the phosphated samples have higher propene selectivity% than phosphate free samples indicating that the phosphate ions increase the acidity of the samples.
Two series of the catalysts under studying have been prepared. The first series is ceria doped alumina catalysts, abbreviated as xCeAl (where, x = 5, 10 and 20 wt%wt ceria) were prepared by coprecipitation method. The 1.5 molar solutions of Al(NO3)3.9H2O and Ce(NO3)3∙6H2O were prepared separately and then mixed in a volume proportions according to the final desired composition of catalysts. The resulting solution was stirred and heated to 350 K in a round bottom flask. The aqueous 1:1 M NH4OH solution was added drop wise to the nitrate solution under vigorous stirring until pH 8 was attained. After ageing for two hours, the excess solution was removed by filtration. Precipitate was washed repeatedly by distilled water followed by drying at 380 K for 12 h. The dried precursor was crushed to fine powder and calcined in the presence of static air at 873 K for 3 h.
The second series, xCeAlP (P refers to phosphate and x is the loading level of ceria, 5, 10 and 20 wt%), was obtained by impregnation of an aqueous solution of the impregnating (NH4)2HPO4 solution and held stirring for 1 h. The solution was adjusted to give 6 Wt% PO4-3 content. The excess water present was removed by evaporation at 380 K. The catalysts were obtained by calcination of the dried samples at 873 K for 3 h.
Apparatus and techniques
Thermogravimetric analysis (TGA) and deferential thermal analysis (DTA) were performed between room temperature and 1273 K in a static atmosphere of air, using Linseis STA PT 1600 thermogravimetric analyzer. The rate of heating was standardized at 10 K /min., and small portions (5–15 mg) of the sample were used in TG measurements.
X-ray powder diffractometry
where K is the crystallite shape constant (≈1), λ the radiation wavelength, β the line breadth (radians) and θ is the Bragg angle.
Nitrogen adsorption isotherm measurement
Full nitrogen adsorption/desorption isotherms at 77 K were obtained using a NOVA 2200, version 6.10 high-speed gas sorption analyzer (Quantachrome Corporation USA). The calcined samples were first out-gassed at 470 K for 1 h. Twenty four-point adsorption and desorption isotherms were obtained, from which BET surface areas were derived using standard and well-established methods (Sing et al. 1985; Webb & Orr 1997).
FTIR spectra for pyrodine adsorption
For pyridine (Py) adsorption on test samples a wafer of 15–20 mg/cm2 were mounted in a Pyrex vacuum cell fitted with CaF2 windows. The samples were pretreated at 700 K for 1 h in stream of O2 followed by evacuation at 700 K for 1 h, then cooled to room temperature to obtain the background IR spectra using Thermo scientific Nicolet 380 FTIR spectrophotometer. Then, 5 Torr Py (1 Torr = 133.32 Pa) were admitted at 300 k for 5 min., degassed for 5 min. at this temperature in order to remove the fraction physisorbed Py, and the spectra were then taken at room temperature.
Catalytic activity (isopropanol decomposition)
The catalytic activity experiments for isopropanol decomposition were carried out in a fluidized bed quartz flow reactor at atmospheric pressure. 0.2 g of the catalyst was activated in-situ at 670 K for 1 h in N2. isopropanol (Merck, product, 99.9%) was introduced at a flow rate of 15 ml min-1 into carrier gas flow of N2. The reaction products were analyzed by gas chromatography on 2-m long 1/8״ column packed with 10% Carbowax and Chromm WHP 80/100 using a model 3400 Varian Gas Chromatograph equipped with a flame ionization detector (FID).
Assoc. Prof. Hussein A. Khalaf: He was born in 1970 in Minia, Egypt. He received a B.Sc. degree of pure chemistry from Faculty of Science, Minia University, Egypt, 1992. He obtained a M.Sc.(1999) and Ph.D (2005) degrees in Physical chemistry (surface and catalysis) from Minia University, Egypt, under the supervision of Professors Seham A. Mansour, Nasr E. Fouad, Ahmed K. Nohman and Gamal A. Mekhemer. And he is currently working as associated prof. at Omar El-Mukhtar University, Libya.
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